1. Field of the Invention
The present invention relates to an optical fiber amplifier. More particularly, the present invention relates to a wideband optical fiber amplifier having an optical fiber doped with a rare earth element, thereby having a capability to amplify optical signals having a wide wavelength band.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is a technique that allows for increases in transmission capacity per optical fiber. The increase in capacity is due to transmitting and receiving a plurality of channels, which are multi-divided to have different wavelengths, through a single optical fiber. Optical communication networks employing the foregoing WDM technique generally make use of C-band or L-band optical signals, in which the C-band has a wavelength band from 1530 nm to 1560 nm, while the L-band has a wavelength band from 1570 nm to 1600 nm. However, due to a sharp increase in communication demands in recently, there have been proposals made with regard to securing a more efficient communication network. More specifically, there had been a proposal to expand a wavelength band of optical signals up to an S-band ranging from 1450 nm to 1500 nm to permit the use of the expanded wavelength band of optical signals.
For the optical communication networks employing the foregoing WDM technique, a wideband optical fiber amplifier is commonly used including an optical fiber doped with various rare earth elements, each of which allows optical signals having different wavelength bands to be amplified. The rare earth elements are exemplified by Erbium, Thulium or the like, ions of which are used for amplification. In particular, ions of Thulium are suitable for amplification of S-band optical signals, which ions of Erbium have difficulty in amplifying.
FIG. 1 shows a prior art configuration of a wideband optical amplifier. According to this prior art, there are a plurality of amplifying units that are arranged in parallel. As shown in FIG. 1, the conventional wideband optical amplifier includes a wavelength band splitting unit 140 for splitting optical signals inputted outside based on each respective wavelength band, wherein a first optical fiber amplifying unit 110 amplifies C-band optical signals. A second optical fiber amplifying unit 120 amplifies L-band optical signals. A third optical fiber amplifying unit 130 amplifies S-band optical signals, and an outputting unit 150 outputs respective amplified optical signals to a single terminal. The C-band optical signals refer to ones having a wavelength ranging from 1530 nm to 1560 nm, the L-band optical signals refer to ones having a wavelength range from 1570 nm to ing1600 nm, and the S-band optical signals refer to ones having a wavelength range from 1450 nm to 1500 nm.
The wavelength band splitting unit 140 divides optical signals inputted into a first terminal into C-band ones, L-band ones and S-band ones Subsequently, the wavelength band splitting unit outputs the C-band, the L-band and the S-band optical signals through second, third and fourth terminals to the respective optical fiber amplifying units. The wavelength band splitting unit 140 includes a sixth wavelength selective coupler 141, a seventh wavelength selective coupler 142, a first isolator 143, a second isolator 144, and a third isolator 145.
The sixth wavelength selective coupler 141 has three ports, of which a first port is connected to the first terminal of the wavelength band splitting unit 140. A second port is connected to the seventh wavelength selective coupler 142, and a third port is connected to the third isolator 145. Accordingly, among optical signals inputted into the first port, the C-band and L-band optical signals are allowed to be outputted to the second port, while the S-band optical signals are allowed to be outputted to the third port.
The seventh wavelength selective coupler 142 has also three ports, of which a first port is connected to the second port of the sixth wavelength selective coupler 141; a second port is connected to the first isolator 143, and a third port is connected to the second isolator 144. Among the C-band and L-band optical signals inputted into the first port of the seventh wavelength selective coupler 142, the C-band optical signals are allowed to be outputted to the second port of the seventh wavelength selective coupler 142, while the L-band optical signals are allowed to be outputted to the third port of the seventh wavelength selective coupler 142.
The first isolator 143 outputs the C-band optical signals, which are outputted from the second port of the seventh wavelength selective coupler 142, to the second terminal of the wavelength band splitting unit 140. However, the first isolator 143 isolates optical signals traveling from the second terminal of the wavelength band splitting unit 140 to the first isolator 143.
The second isolator 144 outputs the L-band optical signals, which are outputted from the third port of the seventh wavelength selective coupler 142, to the third terminal of the wavelength band splitting unit 140. However, the second isolator isolates optical signals traveling from the third terminal of the wavelength band splitting unit 140 to the second isolator 144.
The third isolator 145 outputs the S-band optical signals, which can rang from the third port of the sixth wavelength selective coupler 141, to the fourth terminal of the wavelength band splitting unit 140. However, the third isolator 145 isolates optical signals traveling from the fourth terminal of the wavelength band splitting unit 140 to the third isolator 145.
However, the conventional wavelength band splitting unit has a problem in that it makes use of too many components in order to divide optical signals based on each wavelength band, so that insertion loss of the optical signals is increased before the optical signals are inputted into respective optical fiber amplifying units. The insertion loss means the loss of the entire optical power generated while optical signals pass through a plurality of components, and such insertion loss acts as a factor in deteriorating a noise figure of the wideband optical fiber amplifier.
The outputting unit 150 outputs the S-, C- and L-band optical signals are each outputted from the first and third optical fiber amplifying units 110, 120 and 130 to an exterior through one terminal of the outputting unit 150. The outputting unit 150 includes a eighth wavelength selective coupler 151 and a ninth wavelength selective coupler 152.
The eighth wavelength selective coupler 151 has a first port which is connected with a fourth isolator 111 of the first optical fiber amplifying unit 10 and into which the C-band optical signals are inputted, a third port which is connected with a fifth isolator 121 of the second optical fiber amplifying unit 120 and into which the L-band optical signals are inputted, and a second port to which the C- and L-band optical signals inputted into the first and third ports are outputted.
The ninth wavelength selective coupler 152 has a first port which is connected with the second port of the eighth wavelength selective coupler 151 and into which the C- and L-band optical signals are inputted, a third port which is connected with a sixth isolator 131 of the third optical fiber amplifying unit 130 and into which the S-band optical signals are inputted, and a second port to which the C-, L- and S-band optical signals inputted into the first and third ports are outputted.
The first optical fiber amplifying unit 110 is connected with the second terminal of the wavelength band splitting unit 140 and includes a first erbium-doped optical fiber 114, a first wavelength selective coupler 112, a first pumping light source 113 and a fourth isolator 111, so as to amplify the C-band optical signals inputted from the second terminal of the wavelength band splitting unit 140.
The first pumping light source 113 outputs a first pumping light for pumping the first erbium-doped optical fiber 114. The first erbium-doped optical fiber is pumped by the first pumping light, thereby amplifying the C-band optical signals.
The first wavelength selective coupler 112 has a first port that is connected with the second terminal of the wavelength band splitting unit 140, and a second port that is connected with the first erbium-doped optical fiber 114; moreover, a third port is connected with the first pumping light source 113, and outputs both the C-band optical signals inputted into the first port and the first pumping light inputted into the third port, to the second port connected with the first erbium-doped optical fiber 114.
The fourth isolator 111 is connected with the first erbium-doped optical fiber and the first port of the eighth wavelength selective coupler 151, and outputs the C-band optical signals amplified at the first erbium-doped optical fiber 114 to the first port of the eighth wavelength selective coupler 151. Further, the fourth isolator 111 isolates optical signals traveling from the eighth wavelength selective coupler 151 to the first optical fiber amplifying unit 110.
The second optical fiber amplifying unit 120 is connected with the third terminal of the wavelength band splitting unit 140 and amplifies the L-band optical signals inputted from the third terminal of the wavelength band splitting unit 140. The second optical fiber amplifying unit 120 includes a second erbium-doped optical fiber 124, a second wavelength selective coupler 122, a third wavelength selective coupler 125, a second pumping light source 123, a third pumping light source 126 and a fifth isolator 121.
The second pumping light source 123 outputs a forward second pumping light for pumping the second erbium-doped optical fiber 124, while the third pumping light source 126 outputs a backward third pumping light. Both the forward second pumping light and the backward third pumping light are outputted to the second erbium-doped optical fiber 124. The second erbium-doped optical fiber 124 is pumped by the second pumping light and the third pumping light, thus amplifying the L-band optical signals.
The second wavelength selective coupler 122 has a first port that is connected with the third terminal of the wavelength band splitting unit 140, a second port that is connected with the second erbium-doped optical fiber 124, and a third port that is connected with the second pumping light source 123. The second wavelength selective coupler 122 outputs both the L-band optical signals inputted into the first port and the second pumping light inputted into the third port, to the second port.
The fifth isolator 121 outputs the L-band optical signals amplified at the second erbium-doped optical fiber 124 to the third port of the eighth wavelength selective coupler 151. Further, the fifth isolator 121 isolates optical signals traveling from the eighth wavelength selective coupler 151 to the second optical fiber amplifying unit 120.
The third wavelength selective coupler 125 has a first port which is connected with the second erbium-doped optical fiber 124. A second port is connected with the fifth isolator 121, and a third port is connected with the third pumping light source 126, and outputs the L-band optical signals inputted into the first port to the second port and outputs the second pumping light inputted into the third port to the first port.
The third optical fiber amplifying unit 130 is connected with the fourth terminal of the wavelength band splitting unit 140 and amplifies the S-band optical signals inputted from the fourth terminal of the wavelength band splitting unit 140. Further, the third optical fiber amplifying unit 130 includes a thulium-doped optical fiber 134, a fourth wavelength selective coupler 132, a fifth wavelength selective coupler 135, a fourth pumping light source 133, a pumping module 160 and a sixth isolator 131.
The fourth pumping light source 133 outputs forward fourth pumping light for pumping the thulium-doped optical fiber 134, while the pumping module 160 outputs backward fifth pumping light for pumping the thulium-doped optical fiber 134. The fourth pumping light source 133 makes use of a semiconductor laser with the wavelength band of 1050 nm, or the like as its light source.
The pumping module 160 includes seventh and eighth isolators 161 and 165, a tenth wavelength selective coupler 162, a third erbium-doped optical fiber 164, a fifth pumping light source 163 for pumping the third erbium-doped optical fiber 164, and a separate light source 166 capable of emitting light of the wavelength of 1560 nm. Thus, the pumping module 160 amplifies low power light of 1560 nm, which is outputted from the light source 166, at the third erbium-doped optical fiber 164, thereby outputting fifth pumping light having an intensity enough to pump the thulium-doped optical fiber 134. The light source, 166, may be, for example, exemplified by a distributed feedback laser or the like.
The thulium-doped optical fiber is pumped by the fourth and fifth pumping light, thus amplifying the S-band optical signals.
The thulium-doped optical fiber 134 may make use of the wavelength band of 980 nm or 1560 nm as a pumping light source. However, a high power semiconductor laser having the wavelength band of 1560 nm is still not yet available up to date. For this reason, the pumping light source of the wavelength band of 1560 nm outputs light of the wavelength band of 1560 nm by means of the low power distributed feedback laser or the like. In addition, there must be included a separate pumping module, that is provided with an erbium-doped optical fiber for amplifying light outputted from the distributed feedback laser and a pumping light source for pumping the erbium-doped optical fiber.
The fourth wavelength selective coupler 132 has a first port connected with the fourth terminal of the wavelength band splitting unit 140. A second port is then connected with the thulium-doped optical fiber 134 and a third port connected with the fourth pumping light source, and outputs the S-band optical signals and the fourth pumping light, which are inputted into the first and third port respectively, to the second port.
The fifth wavelength selective coupler 135 outputs the S-band optical signals amplified at the thulium-doped optical fiber 134 to the sixth isolator 131 and outputs the fifth pumping light to the thulium-doped optical fiber 134.
The sixth isolator 131 is connected with the second port of the sixth wavelength selective coupler 135, and the third port of the ninth wavelength selective coupler 152. The sixth isolator 131 outputs the S-band optical signals amplified at the thulium-doped optical fiber 134 to the third port of the ninth wavelength selective coupler 152, but isolates optical signals traveling from the ninth wavelength selective coupler 152 to the sixth isolator 131.
However, the conventional wideband optical fiber amplifier as shown in FIG. 1 divides optical signals based on each wavelength band before the optical signals are inputted into each optical fiber amplifying unit, so that the divided respective optical signals are previously subjected to a decrease in intensity before they are inputted into each optical fiber amplifying unit. Further, before the optical signals are inputted into each optical fiber amplifying unit, the optical signals pass through a plurality of components, which are for splitting the optical signals into ones having each wavelength band. Thus, before the optical signals are inputted into each optical fiber amplifying unit, insertion loss of the respective optical signals is greatly increased. Due to the foregoing factors, the conventional wideband optical fiber amplifier has a problem in that its noise figure and amplification efficiency are deteriorated.